In vitro studies have shown that mechanical factors play an important role in cell cycle kinetics and cell differentiation of the lung through an unknown mechanochemical signal transduction pathway. In this study we evaluated the in vivo role of mechanical factors due to fetal breathing movements (primarily executed by the diaphragm, which is the main respiratory muscle) on lung growth and development by using genetically engineered embryos. Lung growth and development of wild-type, mdx:MyoD+/-(9th) (in which the diaphragm develops normally) and mdx:MyoD-/-(9th) (in which the diaphragm muscle is significantly thinned and not functional) embryos were compared at embryonic day 18.5 using immunohistochemistry, in vivo TUNEL detection and morphometry. No abnormalities in lung organogenesis were observed in mdx:MyoD+/-(9th) term embryos, whereas lung hypoplasia was detected in mdx:MyoD-/-(9th) embryos. In the hypoplastic lung, the number of proliferating lung cells was lower in comparison to the wild-type and mdx:MyoD+/-(9th) embryos, while the gradient of thyroid transcription factor-1 (TTF-1) was not maintained. Surprisingly, no difference was observed in distribution and occurrence of apoptotic lung cells in mdx:MyoD-/-(9th) embryos. Together, it appears that mechanical forces generated by contractile activity of the diaphragm muscle play an important role in normal lung growth and development by affecting cell proliferation and TTF-1 expression.